The present teachings relate generally to ion implantation systems and methods, including systems and methods for adjusting the current density of a ribbon ion beam to enhance its profile uniformity.
Ion implantation techniques have been employed for more than thirty years to implant ions in semiconductors for fabricating integrated circuits. Traditionally, three types of ion implanters are employed for such ion implantation: a medium-current, a high-current and a high-energy implanter. The ion sources incorporated in high current implanters typically include extraction apertures in the form of slots having high aspect ratios in order to ameliorate the effects of space charge. A one-dimensional ion beam extracted from such an ion source can be focused into an elliptical profile to produce a substantially round beam profile at a wafer on which the beam is incident.
Some recent commercial high-current ion implanters impinge a so-called ribbon ion beam, which exhibits a nominally one-dimensional profile, onto a wafer to implant ions therein. The use of such a ribbon ion beam offers several advantages for wafer processing. For example, the ribbon ion beam can have a long dimension exceeding the wafer's diameter and hence can be held stationary as the wafer is scanned only in one dimension orthogonal to the propagation direction of the ion beam to implant ions across the entire wafer. Further, a ribbon ion beam can allow for a higher current at the wafer.
The use of a ribbon ion beam for ion implantation poses, however, a number of challenges. By way of example, a high uniformity of the longitudinal profile of the ion beam is required to obtain an acceptable dose uniformity of the implanted ions. As the wafer sizes increase (e.g., as the next generation 450-mm wafers replace the current predominantly 300-mm wafers), it becomes more challenging to achieve an acceptable longitudinal uniformity of a ribbon ion beam utilized for processing the wafers.
In some conventional ion implantation systems, corrector optics are incorporated into the ion beam line to alter the charge density of the ion beam during ion beam transport. This approach is not, however, generally capable of creating sufficient ion beam uniformity, if the ion beam profile exhibits high non-uniformity upon extraction from the ion source, or due to aberrations induced by space charge loading or by beam transport optics.
Accordingly, there is a need for improved systems and methods for ion implantation, including enhanced systems and methods for generating ribbon ion beams with desired energies and a high degree of profile uniformity.